Line sensors provided with a plurality of light receiving elements that convert incident light into an electronic signal are widely used to, for example, read an image formed on a medium such as a sheet. The reading is typically performed by irradiating the medium with light emitted from a light source and causing light from the medium obtained via the irradiation to be incident on the line sensor. While the reading is being performed, the position relationship between the medium and the line sensor is sequentially changed so that the image of the entire medium can be read.
Even though the image of the same medium is read by causing the light source to emit light under the same condition, the level of a signal output from each light receiving element of the line sensor varies in accordance with, for example, environmental temperature and secular changes in the light source and the light receiving elements. Thus, to avoid a change in a signal level caused by, for example, environmental temperature and a secular change, requirements to drive and cause the light source to emit light are adjusted. In general, in the adjusting method, the light source is caused to emit light under the same condition, the emitted light is reflected from a reflector prepared in advance, the signal level of the light receiving element obtained by causing reflected light from the reflector to be incident on the line sensor is compared with a signal level prepared in advance, and the comparison result is obtained. Using such an adjustment method to adjust the requirements to drive the light source may suppress the change in the signal level of the light receiving elements of the line sensor that would be made by, for example, environmental temperature or a secular change.
Media from which an image is read using the line sensor include banknotes. Some banknotes include a fluorescent substance to prevent circulation of forged notes. The fluorescent substance is applied to a banknote via, for example, printing with fluorescent ink. The fluorescent substance applied to a banknote via printing with fluorescent ink emits light when it is irradiated with excitation light such as ultraviolet rays. Thus, the portion of a fluorescent-substance-including banknote that emits light upon irradiation with excitation light allows a visual inspection to be performed to determine whether the banknote is real or false. Recently, without a visual inspection, a portion that emits light upon irradiation with excitation light has also been able to be read by a line sensor to automatically determine whether the banknote is real or false (patent document 3).    Patent document 1: Japanese Laid-open Patent Publication No. 2001-119530    Patent document 2: Japanese Laid-open Patent Publication No. 2008-287145    Patent document 3: Japanese Laid-open Patent Publication No. 2009-37418